1. Technical Field
The present disclosure relates to wireless transmissions and more specifically to systems and methods of providing a supplemental signal transmission within a coverage hole, such as a region in a broadcast area where interference between a strong local signal and a weak remotely transmitted signal causes the remotely transmitted signal to be difficult or impossible to receive correctly. The supplemental signal carries the same program as the remotely transmitted signal and its transmission in the region proximate to the local signal enables detection of the program in the coverage hole.
2. Introduction
Currently broadcast stations, such as television transmitters, can only share frequency adjacent channel allocations if they are co-located. In the United States, the TV broadcast band is divided roughly as follows: VHF-Low includes channels 2-4 at 54-72 MHz and channels 5-6 at 76-88 MHz; VHF-High includes channels 7-13 at 174-216 MHz; and UHF includes channels 14-51 at 470-698 MHz. Channel 37 at 608-614 MHz is reserved for radio astronomy. This arrangement can vary from region to region. Each television station occupies approximately 6 MHz of bandwidth. For example, assume a station transmits channel 14 between 470-476 MHz and another station transmits channel 15 in an adjacent frequency band between 476-482 MHz. TV stations transmitting these signals can broadcast from a single tower with their respective antennas co-located on the tower. The signal strength of the respective channels in this scenario will generally be equal in radiating distances from the antennas for channels 14 and 15. However it is not always feasible or possible to broadcast all channels from a single location.
FIG. 2 illustrates one prior art approach 200 of broadcasting two signals from separately located broadcast stations. A first TV station broadcasts channel 14 via a first antenna from tower A to a first coverage area 202 and a second TV station broadcasts channel 15 via a second antenna from tower B to a second coverage area 204. In this configuration, the signal strengths of each transmitted signal decline more or less equally with distance away from each transmitter. In regions proximate to one of the broadcast stations, that broadcast station's signal overpowers the other signal, thereby creating a ‘hole’ 206, 208 in the reception of the other signal. Thus, a receiving device 210 in the coverage hole 208 is unable to distinguish channel 15 from channel 14 because the relative signal strength of channel 14 overpowers channel 15. Similarly, a receiving device 212 in the other coverage hole 206 is unable to distinguish channel 14 from channel 15. At point C, a receiving device can only receive the signal from tower A. At point D, a receiving device can receive signals from tower A and B. At point E, a receiving device can only receive the signal from tower B. A receiving device receives and processes the signal to produce an audio program, text, multi-media, television program, and/or some other form of data.
The existence of coverage holes is especially pronounced with frequency adjacent channels. The interference between the two channels A and B is shown by graph 300 of FIG. 3. This graph 300 illustrates the frequency/power correlation for channel A 302 transmitted from tower A and an adjacent channel B 304 transmitted from tower B. As shown in FIG. 3, the short distance between channels A and B further exacerbates interference between the two channels because the frequencies in the roll-off region of the stronger channel B overlap with a portion of the frequencies in channel A.
One way to reduce the interference between channels is to allocate a guard band or channel between the two adjacent channels. Guard bands are used for both terrestrial based communication and satellite communication. FIG. 4 illustrates graph 400 showing the use of a guard band between channels A and B. While this provides some benefit to reducing inter-channel interference, guard bands waste valuable spectrum which could be allocated for other uses. For example, if the tower configuration shown in FIG. 2 were deployed in a city, then the available stations for that city may be limited to every other channel. As another example, the Federal Communications Commission (FCC) may allocate channels 22, 24, 26, 28 and 30 for a certain geographic region and designate channels 23, 25, 27, and 29 as guard bands to prevent interference between channels. As can be seen, as more channels are provided in a market, more guard bands and thus more wasted spectrum must be allocated.
Further, with the advent of modern wireless communications, wireless operators in the US need a significant influx of new spectrum. Existing UHF (ultra high frequency) television broadcast bands are attractive to wireless communications for a number of reasons, but current television broadcast operators fear losing spectrum due to forced reallocation, forced sharing of over-the-air payload between stations, and increased costs for over-the-air service.